In this application, “Optical Communications Technologies” means optical switching, multiplexing transport, network management and access technologies for any format including WDM, packet, and carrier Ethernet as well as the services provided in support of or in connection therewith. Without limiting the generality of the forgoing, Optical Communications Technologies include all of Ciena's current products and services, all products and services currently under development, and all evolutions of such products and services.
Applicant's co-pending U.S. patent application Ser. No. 13/159,871, filed Jun. 14, 2011, the entire content of which is hereby incorporated herein by reference, teaches techniques for distributing a high-bandwidth analog signal to the front end of a multipath analog to digital converter. FIGS. 1A and 1B illustrate principle elements of a receiver module in accordance with U.S. patent application Ser. No. 13/159,871.
Referring to FIG. 1A, the receiver module 2 comprises an electro-optic IC 4 coupled to an electronic signal processor IC 6 via a parallel analog transmission line bus 8. The electro-optic IC 4 includes a 90° optical hybrid 10, a photodetector block 12, and an analog frequency decimation block 14. The optical hybrid 10 receives an incoming optical channel light and a local oscillator light through respective pigtails 16 and 18, and operates in a conventional manner to mix the two lights together to generate composite lights that are made incident on the photodetector block 12. Similarly, the photodetector block 12 operates in a conventional manner to generate an analog photodetector signal V that is proportional to the power of the incident composite light. The frequency decimation block 14 processes the photodetector signal to yield a set of parallel analog signals Vx (where x is an index value, x=1 . . . N) which, when taken together, contain all of the information content modulated on the photodetector signal V; but which, taken individually, have a lower bandwidth than the photodetector current V. The electronic signal processor IC 6 comprises analog signal conditioning circuits (such as power amplifiers, filters etc., not shown) and analog-to-digital (A/D) converters 20 for converting the analog electrical signals Vx from the frequency decimation block 14 into raw digital sample streams which are processed by the DSP 22 to reconstruct the spectrum of the photodetector signal V and recover digital data signals modulated on the received optical channel signal
As is known in the art, a conventional 90° optical hybrid is configured to mix the received optical channel light with the LO light and a 90° phase-shifted version of the LO light, to generate corresponding In-Phase and Quadrature composite lights for each of two polarizations of the incoming optical channel light. In many practical embodiments, it is desirable to provide respective parallel signal paths (each comprising a photodetector 12 and an analog frequency decimation block 14) for receiving and processing each of these composite lights. However, for simplicity of illustration, only the In-Phase signal path for a single polarization is shown in FIG. 1A, it being understood that the signal path(s) for the corresponding Quadrature composite light, and for the second polarization (if any), could be provided by suitably duplicating the elements of the In-Phase signal path.
Referring to FIG. 1B, a representative frequency decimation block 14 comprises an analog 1:N power splitter 24, which receives the photodetector current V, and outputs a set of N parallel duplicates of the photodetector current V in a known manner. In the illustrated embodiment, N=4, but this is not essential. Increasing the number N of outputs reduces the bandwidth performance requirements of the analog transmission line bus 8, at the cost of increased complexity. For enhanced performance the splitter 24 may contain filtering and or preamplification functions which, for simplicity of illustration, are not shown in the drawings. Each output of the 1:N splitter 24 is connected to a respective analog signal path, each of which includes a respective non-linear processor 26a-d cascaded with a low-pass filter (LPF) 28a-d. Each non-linear processor 26 applies a non-linear operation to the photodetector current V using a respective branch signal Bx to yield a composite signal VBx that is supplied the LPF 28. The LPF 28 operates in a conventional manner to attenuate undesired high-frequency components to yield a low bandwidth analog signal Vx, which can be transmitted through the analog transmission line bus 8 to the electronic signal processing IC 6.
The non-linear processors 26a-d can be designed to implement any suitable non-linear operation. For example, in the embodiment of FIGS. 1A-1B, the non-linear processor 26 is implemented as a conventional Radio Frequency (RF) mixer, which operates to combine the photodetector current V and the respective branch signal Bx in a known manner. In an embodiment in which the branch signals Bx are continuous wave sinusoidal signals, the non-linear function is the well known heterodyne or homodyne function. In embodiments in which the branch signals Bx are binary digital signals, the non-linear function approximates a switching or sampling function, depending on the duty cycle of the branch signals Bx. In either case, each parallel analog signal Vx (x=1 . . . N) is an analog signal having a bandwidth determined by the respective LPF 28. After sampling these signals by the A/D converters 20, it is desired to recombine the signal paths in the DSP 22 to recover a digital representation of the original photodetector current V, which is sufficiently accurate that data modulated on the received channel light can be detected and recovered.
One method by which the signal paths may be recombined in the DSP 22 is to digitally process each signal, downstream of the A/D converters 20 so as to reverse the effects of the non-linear processors 26. For example, in a case where a given non-linear processor 26 implements a conventional down-conversion function, the corresponding digital signal may be digitally up-converted to offset this effect. The digital signals may then be filtered to remove undesired reflected images, and then combined using a digital summation process to yield a high-bandwidth digital signal corresponding to the original photodetector current V. A limitation of this approach is that it may significantly increase the size and cost of the DSP 22.
Techniques that overcome limitations of the prior art remain highly desirable.